Biaxially stretched polyester films have excellent mechanical, thermal and electrical properties and chemical resistance, and are used in various fields. Particularly, they are rivaled by no other type of polymer films as the base film for magnetic recording tapes. The quality of base films for magnetic recording tapes has a great influence upon the quality of the final product magnetic tapes. Due to constant advances in the magnetic recording tape industry, there is increasing demand for base films of higher quality.
For instance, properties of magnetic recording tapes such as output, S/N ratio, dropout, generation of envelope are related to flatness of the base film, and the shape or appearance of a wound-up tape, and slidability and wear resistance are closely related to slipperiness of the base film. Therefore, polyester films provided with excellent flatness and slipperiness are required for production of magnetic recording tapes, especially those for video recording.
On the other hand, thinner magnetic recording tapes are desired, since magnetic recording apparatus is lightweight and more compact and has great capacity for longer periods of recording time. For such thin base films, polyester films with high longitudinal tensile strength have been used. These films are produced by stretching ordinary biaxially oriented films further longitudinally. As a larger variety of magnetic recording tape procuts are produced today, there is increasing demand for polyester films which have high strength, not only longitudinally but also transversely. That is, there has been a strong desire for base films with high biaxial strength as well as excellent flatness and slipperiness properties.
The conventional method for providing polyester films with good slipperiness properties is to roughen their surfaces with externally added or internally deposited fine particles. By this method, however, flatness of the film surface is impaired if the polyester film is produced under the conditions ordinarily employed today, and therefore it is not suitable for production of high quality magnetic recording tapes. Moreover, when a biaxially oriented film is further stretched longitudinally and/or transversely, there is a tendency for slidability characteristics to remarkably deteriorate, although the film surface does become flat. Thus, it has been considered that flatness and slipperiness of the film are incompatible characteristics, since it is difficult to obtain high strength films satisfactory in both properties.
We have conducted intensive research in order to obtain high strength polyester films excellent in flatness and slipperiness, and have found that the problem is solved by stretching a film longitudinally in multiple stages under conditions that restrict the degree of orientation, then stretching the thus obtained longitudinally stretched film transversely, and further stretching the thus obtained biaxially oriented film longitudinally and/or transversely.